When series capacitors are used in an electric utility transmission system to reduce the reactive component of line impedance, an oscillatory electric circuit is created that has a natural electrical frequency in the subsynchronous frequency range, generally between 10 and 40 Hz, as contrasted to the power generating operating frequency of 60 Hz. Transient currents in the subsynchronous frequency range interact with the field excitation of the generator rotor of the power generating equipment to produce pulsating air-gap torques, at slip frequency, that is, between 30-50 Hz. The slip frequency air-gap torques produce rotor oscillations and corresponding torques on the shafts connecting the generator and turbine rotors of the power generating equipment. If the slip frequency torque happens to correspond closely to any of the mechanical torsional natural frequencies of the turbine generator assembly, the resulting rotor motions of the power generating equipment can become appreciable and the corresponding shaft torques that develop can be damaging and, as for example, may cause breakage of the shaft. Furthermore, the generator rotor motion produces a voltage in the armature circuit which reinforces the electrical system natural frequency currents so that they decay more slowly than they would for constant rotor speed. In the limit these coupled mechanical and electrical system oscillations reinforce each other to the point where they become unstable, in which case they will build up spontaneously to damaging levels without any large initiating system distrubance. This is the phenomena known as subsynchronous resonance (SSR).
It is highly desirable to maintain continuing operation of a power generating system, although such power generating system is subjected to widely varying conditions over a period of time. The varying conditions include fault currents, switching operations, varying loads and varying voltage-current phase relationships. In general, it has been found that long-distance power transmission systems, and the loads thereon, are inductive in nature and it is advantageous to compensate for this by introducing series capacitor compensating means into the power transmission system for the purpose of improving stability or voltage regulation or obtaining the desired current flow division.
U.S. Pat. No. 3,813,593 discloses a method for reducing turbine-generator shaft torques wherein a plural section power filter is serially connected in the power line of the power transmission system between a low voltage side of each winding and a common neutral in each leg of the Y secondary of the generator step-up transformer. The filter is tuned such that it offers a minimum impedance to current at the power frequency while presenting an impedance to current at subsynchronous frequencies in a ratio proportional to the amount of stress expected to be caused to the turbine generator connecting shaft by the interaction of subsynchronous frequency currents with the generator magnetic flux.
Another method currently employed for reducing turbine-generator shaft torque consists of connecting a transformer directly to the generator terminals, the secondary of which has connected a reactor and a pair of thyristors between each of the transformer secondary windings. A control circuit senses the occurrence of subsynchronous current and fires the thyristors to provide an opening current to dampen the sybsynchronous currents.
The devices employed in the combination of thyristors and reactor connected across each of the generator terminal phases requires a considerably higher mVA rating than the devices associated with the series-thyristor SSR damper, and the former is, therefore, more expensive than the latter.
The purpose of this invention is to provide a means for attenuating subsynchronous frequencies in a power transmission system utilizing an arrangement of circuit elements that are less costly than those of the prior art.